1. Field of the Invention
Thermally generated density differences in a fluid (e.g. in a gas) are already used in known sensors for detecting movement.
2. Description of the Related Art
U.S. Pat. No. 4,232,553, for example, discloses an arrangement for measuring angular acceleration by means of a fluid-filled toroid, where two opposing pairs of heating and sensor elements can detect a relative movement of the fluid with respect to the rotating toroid.
The German Patent of the applicant, DE 4243978 C1, filed on 23.12.92, describes an inclination and acceleration sensor which uses the convective flow in a fluid to obtain movement-dependent signals and can be manufactured using microsystems technology.
Movement of sensor elements in an isothermal field is also used in the sensor to detect a rotational movement or a rotational acceleration according to EP 1 111 395 A1.
The EP 0 674 182 B1 describes a hybrid-sensor, in which a gas is in a hermetically closed space. This sensor also uses thermal effects to detect acceleration and angular velocity. By an assembly of heat wire resistors the static distribution of temperature and the deviation of gas flow are detected.
In the sensors described it is taken as the starting point that a statically built-up isothermal field (i.e., a stable, nonturbulent convection flow) in a homogeneous fluid is displaced or altered by the movement. Since the measurement structures in these sensors must only adapt thermally (e-function), the reaction times are naturally relatively long and thus the applications are preferably to be seen in low-frequency measurement cycles.
In addition, a relatively high operating power is required to maintain the convection flow or the static isothermal field which further restricts the possible applications of these sensors.
The US 2003/0074953 A1 discloses a device for determining the change in density of a solid, liquid or gaseous medium. The device comprises a emission device and a receiving device coupled with the medium. The medium itself is not a component of the device. Transmitter, receiver and medium are spatially separated from each other and they should be aligned to each other in a certain position.
The measured changes in density of the medium are caused by physical and/or chemical effects. The transmitter itself did not generate them.